Ibn al-Haytham’s book on optics showed how to make progress in understanding nature

Eleventh-century scholar Ibn al-Haytham, known as Alhazen, was a major contributor to the field of optics and one of the most curious characters in the history of medieval science. He was commemorated on this Iraqi banknote.

When it comes to celebrating scientific anniversaries, there aren’t many opportunities to look back a whole millennium. Western Europe in the 11th century was about as scientifically astute as an anti-vaccine convention today. But farther east, in the Islamic world, numerous savants were hard at work preserving and expanding the wisdom of the ancient Greeks.

Soon after the advent of Islam, scholars had begun translating the treatises of Aristotle and other great philosophers and mathematicians into Arabic. In the centuries that followed, Arabo-Islamic intellectuals extended their efforts far beyond mere translation of ancient texts. Science flourished.

Arabic writers wrote commentaries on the Greeks, developed mathematical advances such as algebra and investigated key questions on which ancient thinkers disagreed, such as the architecture of the cosmos and the nature of vision. Among the greatest of such scholars was Alhazen, the Latinized name for Abū ‘Alī al-Hasan Ibn al-Hasan Ibn al-Haytham.

His greatest work, Kitāb al-Manāzir (Book of Optics), was composed sometime around 1015. That’s among the reasons that the United Nations, in a stark rebuke to vampires and followers of Lord Voldemort, has declared 2015 to be the International Year of Light.

A millennium ago, optics was not merely a subfield of physical science, pursued for its benefits to photography, spy satellites or high Internet speeds via optical fiber. Optics was one of the most important and essential of the sciences, much in the way cosmology or particle physics is viewed today. Many ancient scholars regarded light as the most fundamental of corporeal substances, and vision was the prime sense for knowing about nature, the pathway by which external reality touched the internal soul.

Alhazen was undoubtedly the most significant figure in the history of optics between antiquity and the seventeenth century.

— David Lindberg

And so Ibn al-Haytham, the greatest optical thinker of the age, clearly warrants millennial-style celebrating.

“Alhazen was undoubtedly the most significant figure in the history of optics between antiquity and the seventeenth century,” historian of science David Lindberg wrote in a classic book on theories of vision.

Even apart from his enormous contribution to optics, Ibn al-Haytham was one of the most curious characters in the history of medieval science. Accounts of his life, available in writings from 13th century Muslim scholars, are somewhat garbled and inconsistent. But he appears to have been a rather clever character. Born in Basra (present day Iraq), he was clearly well-educated, having mastered the writings of nearly all the famous Greek authors — Aristotle and Plato, Ptolemy, Archimedes, Galen and many others. Ibn al-Haytham was an accomplished natural philosopher, mathematician and astronomer, and produced by some estimates more than 200 books on various aspects of nature.

By one account, Ibn al-Haytham had bragged that he knew how to build structures that could control the flow of the Nile River. The caliph of Cairo at the time heard of Ibn al-Haytham’s boasts and invited him to Egypt — and then ordered him to proceed down the Nile and put his money where his math was. Apparently Ibn al-Haytham did travel down the Nile, but soon realized his plan had a snowball’s chance in the Sahara of actually working. Upon his return to Cairo, he feigned insanity to deflect the caliph’s ire and spent many years under house arrest until the caliph died.

By other accounts, Ibn al-Haytham employed the fake insanity ploy in Iraq to avoid a job he didn’t enjoy. But whatever his life circumstances, he found a way to write about many of the major scientific questions of the day, especially the relationship between light and vision.

Ancient authorities differed dramatically on that question. Euclid and Ptolemy, modifying the views of Plato, insisted that vision required an emission from the eyes to the perceived object. Followers of Aristotle argued to the contrary. Light from radiant sources allowed images of objects to reach the eye through transparent media (like air), the Aristotelians believed. Variations of that dichotomy emerged from students of Galen, who had described the physiology of the eye and optic nerves leading to the brain — where the soul, supposedly, absorbed the imagery that light and vision provided.

Ibn al-Haytham sided mostly with Aristotle’s “intromission” view, that vision involved the eye’s reception of external images. Extramission — light emerging from the eye to achieve vision — seemed hard to reconcile with some obvious objections. Aristotle himself pointed out that it would be unreasonable to suppose that an eye could emit rays capable of reaching all the distant stars.

But Ibn al-Haytham did not stop with dissing extramission. He also had to explain why the image of, say, a mountain, could fit within the relatively tiny human eyeball. In that regard, Euclid and Ptolemy had worked out elaborate geometrical-mathematical descriptions of how rays from the eye could create a visual cone able to encompass images of the objects that the eye perceived. Ibn al-Haytham saw how that math could be applied to “imaginary rays” passing into the eye from various points on a perceived object. In other words, the geometry that extramission advocates had applied to supposedly physical emitted rays of light could be turned around to describe the mechanics behind received rays of light. Combining these insights with his knowledge (via Galen) of the eye’s physiology, Ibn al-Haytham further described the visual process (noting the importance of the lens) and how it sent images to the brain.

Ibn al-Haytham’s theory was not entirely correct. Only with Kepler’s work, in the 17th century, did the modern understanding of images projected onto the retina clearly emerge. But Ibn al-Haytham's underlying philosophical strategy paved the way to that modern understanding. Whereas previous scholars had argued for one or the other of competing views — whether the physical (Aristotle), mathematical (Plato-Euclid-Ptolemy) or physiological (Galen), Ibn al-Haytham sought to merge those superficially irreconcilable ideas. His program was one of unification of different ways of knowing.

“Alhazen’s commitment to a theory of vision that combines the physical, the physiological and the mathematical has defined the scope and the goals of optical theory from his day to the present,” Lindberg wrote.

Ibn al-Haytham’s achievement was greater than it might seem today, when math and physics go together like Belichick and Brady. In medieval times, math and physics served distinct purposes, not only in optics but also in astronomy. Physicists (or in the terminology of the times, natural philosophers) sought physical explanations for the workings of nature. Mathematicians sought formulas for describing what happened in nature. Mathematicians could predict where the planets would be as they moved through the sky; natural philosophers proposed ways to explain how such motion was physically possible. But the mathematical descriptions of the heavens that predicted the motions of planets and stars were utterly incompatible with the prevailing physical understanding about how physical processes could produce astronomical motions. Ibn al-Haytham insisted on the need to reconcile the conflicting approaches.

Ibn al-Haytham not only appreciated the importance of merging the math with the physics. He also understood the need to merge science with society. Knowledge was treasured in the medieval Muslim world, for everything from timekeeping to casting horoscopes to building mosques with the right orientation toward Mecca. Science served those societal needs, and in the Arabo-Islamic culture of the early Middle Ages, science was “practiced on a scale unprecedented in earlier or even contemporary human history,” as historian Ahmad Dallal has written.

Ibn al-Haytham was only one in a long line of Muslim philosophers, astronomers and mathematicians who helped to build the foundations for science as it is practiced throughout the rest of the world today. So celebrating the millennial anniversary of Ibn al-Haytham’s optics should be viewed as a celebration of all Arabo-Islamic science and the debt the modern world owes it. And also, perhaps, it should be a time to reflect on what happens when science’s role in society is severely diminished, as happened in the Arabo-Islamic world centuries ago and is evermore so frequently threatening to happen in America today.